1. Field of the Invention
The present invention relates to a method of producing solid moldings which can be effectively used as building materials from two ingredients: a slurry which is a by-product from an apparatus for removing sulfur dioxide gas (hereinafter referred to as "SO.sub.2 ") by absorption from combustion gas using limestone as an absorbent, and a flyash obtained from a coal-fired combustor.
The present invention also relates to a method of producing solid articles which can be used as building materials from a slurry drawn from a wet limestone-gypsum-process flue-gas desulfurization apparatus after the absorption process.
2. Description of the Related Art
A wet limestone-gypsum-process flue-gas desulfurization apparatus uses a slurry in which limestone is suspended to absorb SO.sub.2 from flue gas with an attendant neutralization reaction. It thus can achieve a high rate of SO.sub.2 absorption and is in wide use as an excellent desulfurization method which can be done with very compact equipment and which is inexpensive in operation. However, there are problems inherent to an apparatus of this type. They are the treatment of the slurry after absorption and that of the waste water.
The former has been solved generally by separating the slurry into solids and liquid using a centrifuge and by thus taking out gypsum, the principal component of the slurry, as a solid product. The gypsum is formed into plasterboard for walls or used as an additive to cement. A common solution for the latter has been setting up waste water treatment installations. Effluents from industrial processes and the like generally contain heavy metals and COD-increasing substances. From the viewpoint of environmental protection the waste water treatment facilities for the elimination of these deleterious substances are essential. Also, for the purposes of process operation the accumulation of impurities in the absorbent slurry which would result in a decrease in the SO.sub.2 absorptivity has to be prevented. Thus, it is necessary to discharge spent liquid from the system and accordingly treat the waste liquid.
FIG. 3 is a schematic diagram illustrating a typical arrangement for the conventional treatment of slurry and waste water. A conventional process for treating slurry and waste water will be explained below with reference to this drawing. SO.sub.2 -containing flue gas is introduced via line 301 into an absorption column 302, where it comes in contact with a circulating slurry. A tank 303 at the bottom of the absorption column is supplied with air through line 304 to oxidize calcium sulfite in the slurry to gypsum.
The slurry in which gypsum particles and limestone particles are suspended is forced out and up from the bottom tank 303 of the absorption column by an absorption-column circulating pump 305. It is then sprayed through spray pipes 306 to absorb SO.sub.2 from the flue gas. The treated gas is discharged from the system by way of line 307. Limestone particles which are an ingredient for the absorbent are fed through line 309 to a material preparation tank 308, where they are mixed with a filtrate supplied through line 310 to form a slurry. The slurry is supplied via line 311 to the tank 303 at the bottom of the absorption column. The slurry circulating through the absorption column is partly drawn out through line 312 into a solid-liquid separator 313, where gypsum 314 is recovered as a by-product.
A part of the filtrate from the solid-liquid separator 313 is conducted through line 315 into a first thickener 316 which is a component unit of a water-treatment system. To the first thickener 316 added through line 317 is a Ca(OH).sub.2 slurry as a pH adjuster so as to remove suspended solids, such as flyash, and also precipitate and remove metallic constituents as hydroxides. The suspended solids and heavy metal hydroxides are discharged together as a sludge 319 from the first thickener 316 via an underflow line 318 out of the system. Overflow liquid from the first thickener 316 is led through line 320 into a second thickener 321, where an Na.sub.2 CO.sub.3 solution is added by way of line 322 in order to precipitate for removal the calcium that can cause troubles associated with hard steam in the subsequent treatment steps. The precipitate is discharged as a sludge 324 from the second thickener 321 via an underflow line 323 to the outside.
An overflow from the second thickener 321 is led through line 325 to a thermal decomposition tank 326, where it is mixed with sulfuric acid supplied via line 327 and heated together. In this way the COD-increasing components, mainly dithionic acid, are separated from the absorption liquid and then, after the adjustment of pH with caustic soda added via line 328, the liquid is drained from the system.
In another example of conventional process shown in FIG. 4, untreated flue gas 401 is introduced into an absorption column 402 where it is freed from sulfur by contact with a spray of circulating absorption slurry which is forced out of an array of spray pipes 406 in the upper space of the column 402 by a pump 405. It is then discharged as clean gas 407. The circulating absorption slurry that has absorbed SO.sub.2 from the flue gas and flown down into a circulating tank 403 contains the absorbed SO.sub.2 in the form of a sulfite. In order to oxidize this sulfite to gypsum, air 404 is injected into the circulating tank 403 at the bottom. Meanwhile, line 411 supplies the circulating tank 403 with a limestone slurry absorbent in an amount adequately proportional to the stoichiometric amount of the SO.sub.2 absorbed. An amount of the circulating absorption slurry corresponding to that being supplied is drawn out via line 412 into a filtrate and a solid-liquid separator 413, where it is separated into a solid phase 414 consisting mainly of gypsum. Part of the filtrate is conducted through line 410 to a limestone slurry storage tank 408 for the preparation of a limestone slurry, while the remainder is discharged from the system by way of line 415 to a waste water disposal station 416. Limestone is fed through line 409 to the limestone storage tank 408.
In conventional processes, solids are separated from the slurry after absorption, and the resulting gypsum is utilized, e.g., in the form of plasterboard or an additive to cement. However, local environments and situations have sometimes made it impossible to maintain an adequate balance between the demand and supply of this material. Thus, recovered gypsum has not always been effectively utilized; some part of it has often to be disposed of at landfill. The sludge that results from the treatment of waste water has to be abandoned also, while a market for its effective use has been sought.
Another problem has been that, as FIG. 3 indicates, the waste water treatment involves such complex process steps that it requires much manpower for the operation control and a fairly large land for the placement of the equipment.
In the case of the flue gas coming from a coal-fired combustor, a large volume of flyash is recovered from it by a dust collector. At present, a small percentage of the flyash is utilized as cement material and the like but most is abandoned for landfill. In some regions the shortage of available landfill is a serious problem.